Since writing Reading the Comments I often think about how to best explain why it is people can act so rotten online.
I recently put together a graphic that uses the “bad apple” idiom.
The three sources of rottenness also, roughly, follow the development of theories about online behavior.
However, I believe the temperature (media effects), state of the barrel (group culture), and presence of worms (disordered personalities) are all still relevant.

When researchers first started talking about flaming back in the 90s, they tended to focus on the effects of “Computer-Mediated Communication” (CMC).
Researchers spoke of reduced social cues, media richness, and social information processing; they offered theories of hyperpersonal media and of deindividuation effects.
When considering why people can be so rotten, I think these media effects are related to the effect of temperature on apples.
The hotter it is, the faster the apples in a barrel will spoil.
We can see this when Lindy West’s cruelest troll apologized: “it finally hit me. There is a living, breathing human being who is reading this shit. I am attacking someone who never harmed me in any way.”
Interacting online had distanced him from the consequences of his actions.
Using digital communication can increase the temperature and the likelihood of something rotten happening.

In the new millennium, these media effects were supplemented by a focus on environment and culture.
Watt, Lea, and Spears wrote that “theoretical revisions have moved away from the central importance of communication bandwidth.”
They also argued that people still have inhibitions and norms when online, people just look to more salient norms.
Trolling had become its own culture, with its own norms.
As Coleman wrote in 2011: “trolls have transformed what were more occasional and sporadic acts, often focused on virtual arguments called flaming or flame wars, into a full-blown set of cultural norms and set of linguistic practices.”
In this light, someone like Violentacrez didn’t become wholly uninhibited by norms; he was doing what had become the norm in his corner of Reddit.
Just as an apple in a rotting barrel is likely to go bad, someone hanging out in a rotten subreddit is more likely to do the same.

Finally, although folks have long been armchair-diagnosing others, researchers are beginning to consider personality.
Buckels, Trapnell, and Paulhus found measures of sadism, psychopathy, and Machiavellianism are positively correlated with trolling and that there was a strong relationship between “online commenting frequency, trolling enjoyment, and trolling behavior and identity.”
Although I object to using the term “troll” to label any undesirable behavior, I relate this to the worm that spoils an apple.
Maybe folks like Violentacrez and weev would test highly on this “dark tetrad” of personality variables.
At the extreme, I sometimes reference Luka Magnotta, who was diagnosed with paranoid schizophrenia in his teens.
As I wrote in Reading the Comments, in his twenties he became notorious for his online exploits, including suffocating cats; he eventually killed and dismembered a man—all posted on YouTube.
In 2012 he fled to Europe where he continued posting videos taunting police and thanking “his fans” for their attention and support.
Magnotta was eventually arrested in an Internet cafe in Germany reading stories (and likely commenting) about himself.
Although disordered folks are a minority, they can have a disproportionate effect, especially online.

All of these things, temperature, environment, and worms contribute to a rotten barrel of apples.
Similarly, media effects, culture, and the disordered do the same online.